1. Field of the Invention
This invention relates to a subharmonic noise reduction circuit for improving the signal-to-noise ratio characteristic of an electrical circuit having inherent subharmonic distortion.
2. Description of the Prior Art
In order to reduce subharmonic noise with conventional circuits, it is necessary to use Fourier transforms which require relatively expensive hardware and/or software, as well as excessive power. The subharmonic noise reduction circuit according to this invention permits use of a single low cost sensor over a wide bandwidth and thus the output of the circuit can be easily processed using time domain in lieu of the more complex frequency domain analysis associated with conventional noise reduction circuits.
U.S. Pat. No. 4,622,692 relates to a noise reduction circuit for enhancing a speech signal corrupted by low-frequency noise. A microphone is used to produce an audio-frequency electronic speech signal which represents speech sounds incident upon the microphone. The circuit includes a high-pass channel, a broadband channel and a summing device, such as an operational amplifier, which combines signals received from the high-pass and broadband channels.
U.S. Pat. Nos. 4,441,083 and 4,441,084 each disclose a noise reduction circuit. Both patents identify problems associated with temperature dependent transistors, which are difficult to control, since a direct current shift may occur as result of the changing resistance of the transistor, as a function of temperature. Both patents further suggest that in addition to the temperature dependent transistors, it is difficult to obtain a variable resistance element having precise characteristics and thus the dynamic range of a noise reduction circuit can only be increased by approximately 10 to 20 dB. The '083 patent discloses a noise reduction circuit which can be used as an encoder circuit at the input side of an audio signal recording and/or reproducing apparatus for effecting a level compression operation. The '084 patent discloses a noise reduction circuit which can be used as a decoder circuit on the output side of an audio signal recording and/or reproducing apparatus for effecting a level expansion operation.
U.S. Pat. No. 4,733,193 discloses a circuit for eliminating signal hum. The circuit detects a signal present on the output of the whole of the arrangement and the signal from a feedback loop which has a feedback factor less than 1. On the basis of such signals, the circuit is used to decide whether the useful component of the signal applied to the input of the arrangement is, compared to the hum component of the signal, of sufficiently low strength to be considered negligibly low. If found negligibly low, a separate circuit initiates a control signal sequence which corrects the hum waveform downstream of the feedback loop.
U.S. Pat. No. 4,811,422 discloses a circuit for reducing the amplitude of undesired radio frequency harmonics. Such circuit is particularly suitable for attenuating harmonics in AM, FM, TV, radar transmitters and other high powered transmitters. The circuit has a path which includes a power amplifier for the undesired radio frequency harmonics and causes the harmonics in such path to have correct amplitude and phase to substantially cancel the undesired harmonics when they are combined with a main signal in a normal path.
U.S. Pat. No. 4,560,945 discloses a feedforward cancellation system having a plurality of inputs for receiving a plurality of fundamental tone signals, a power bank amplifier, and adaptive and passive feedforward cancellation circuits. U.S. Pat. No. 4,739,280 discloses an amplifier circuit having two general purpose operational amplifiers for reducing crossover distortion.
U.S. Pat. No. 4,812,669 discloses a harmonic suppressing device having a high-order higher harmonic filtering unit, such as a passive filter, which absorbs higher harmonic currents of higher orders among the higher harmonic currents generated by a higher harmonic current source, and a low-order higher harmonic filtering unit, such as an active filter. U.S. Pat. No. 3,918,005 discloses a circuit configuration of two tandemly connected operational amplifiers in which either controlled current or controlled voltage is applied at a load impedance connected across the inverting input and the output of a second operational amplifier. Enhanced voltage compliance at the load impedance is produced by feeding back an inverted portion of the output signal from the operational amplifier to its non-inverting input and by feeding back a portion of the operational amplifier output signal to the input of the control current signal source. The resulting enhancement is by nearly a factor of two.
U.S. Pat. No. 4,446,447 discloses a circuit for converting the variation of an electrical capacitance, caused by pressure variation, to a frequency variation in a periodic electrical signal generated by the circuit. A digital mixer receives first and second periodic signals as inputs and provides an output signal which is the difference between the first and second periodic signals.